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**JetePlentuara** · 08-04-2012, 10:59 AM

There are certain circumstances where that's allowed. In medical research this approach is usually expressly forbidden, because it leads to hypothesis drift - the idea of flipping through multiple hypotheses until you find one that fits your data in such a way as to make it statistically-significant. Because this makes the hypothesis another variable and changes the truth value, it's generally at the very least frowned-upon.

It's my understanding that you should only do a one-tailed test when you have prior evidence to believe that a one-tailed test is warranted. But what if you were trying to figure out something that had no prior research? Say for example what proportion of FM members preferred coke and what proportion preferred pepsi? Obviously there is no prior evidence to go on, so you would only be able to do a two-tailed test in this situation and all you could ever say was if the proportion liking coke was equal or not equal to the proportion liking pepsi. But you could never say the proportion liking coke > or < the proportion liking pepsi?

08-04-2012, 10:59 AM	#5
JetePlentuara Join Date Oct 2005 Posts 414 Senior Member	There are certain circumstances where that's allowed. In medical research this approach is usually expressly forbidden, because it leads to hypothesis drift - the idea of flipping through multiple hypotheses until you find one that fits your data in such a way as to make it statistically-significant. Because this makes the hypothesis another variable and changes the truth value, it's generally at the very least frowned-upon. It's my understanding that you should only do a one-tailed test when you have prior evidence to believe that a one-tailed test is warranted. But what if you were trying to figure out something that had no prior research? Say for example what proportion of FM members preferred coke and what proportion preferred pepsi? Obviously there is no prior evidence to go on, so you would only be able to do a two-tailed test in this situation and all you could ever say was if the proportion liking coke was equal or not equal to the proportion liking pepsi. But you could never say the proportion liking coke > or < the proportion liking pepsi?
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